by NASA: While our nation's satellites rapidly age, we're losing our ability to observe and respond to dangerous environmental threats. NASA

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The American Association for the Advancement of Science calls it a crisis. Atmospheric scientist Timothy L. Killeen, the president of the American Geophysical Union, says it "could harm our ability to protect our citizens." We call it plain old scary. It's the endangered future of our nation's arsenal of Earth-observing satellites, the 42 instruments that enable scientists to monitor the planet. Satellite images and colorful data sets like the ones on the following pages help researchers track killer hurricanes, plan conservation efforts, manage water resources, and predict glacial melting.

Yet our fleet is aging, and badly. A recent National Research Council report predicts that by 2010, the number of working satellite sensors will drop by 40 percent. Even more troubling, it warns that the next generation of sensors won't be able to adequately address the ever more intricate questions that scientists will pose about the land, sky and oceans over the next decade.

Blame in part the shifting priorities of NASA, one of the main agencies that manage our satellites. NASA's earth-science budget decreased 30 percent between 2000 and 2006. Although the space agency has requested a $1.1-billion increase over 2007, most of its budget is going to fund the International Space Station and President Bush's Vision for Space Exploration, a plan to send humans back to the moon by 2020 and, ultimately, to Mars.

We support sending humans to space, but let's not forget where we live now. The U.S. needs a timely recommitment to Earth observation. Otherwise, our ability to monitor, predict, and respond to dire environmental threats will continue to erode.

To see what our satellites reveal about the changing planet, launch the gallery here.